1. Field of the Invention
This invention relates to a method and apparatus for performing ophthalmic surgery, and more particularly to a method and apparatus for performing ophthalmic surgery using a guided laser beam.
2. Description of Related Art
The use of laser beams to surgically alter the human eye is well known today, and is based upon the concept of correcting refractive errors by changing the curvature of the eye. This concept was brought forth early on, as illustrated in the notable mechanical methods pioneered by J. Barraquer. These mechanical procedures involve removal of a thin layer of tissue from the cornea by a micro-keratome, freezing the tissue at the temperature of liquid nitrogen, and re-shaping the tissue in a specially designed lathe. The thin layer of tissue is then re-attached to the eye by suture. The drawback of these methods is the lack of reproducibility and hence poor predictability of surgical results.
With the advent of lasers, various methods for the correction of refractive errors have been attempted, making use of the coherent radiation properties of lasers and the relative precision of the laser-tissue interaction. Peyman, et al., in Ophthalmic Surgery, vol. 11, pp. 325-9, 1980, reported that laser burns of various intensity, location, and pattern were produced on rabbit corneas. More recently, Horn, et al., in the Journal of Cataract Refractive Surgery, vol. 16, pp. 611-6, 1990, reported that a curvature change in rabbit corneas had been achieved with a laser by applying specific treatment patterns and laser parameters.
In U.S. Pat. No. 4,907,586 to Bille et al., a technique for tissue ablation of the cornea is disclosed in which a laser beam is focussed into a small volume of about 25-30 microns in diameter. The peak beam intensity at the laser focal point could reach about 10.sup.12 watts per cm.sup.2. At such a peak power level, tissue molecules are "pulled" apart under the strong electric field of the laser light, which causes dielectric breakdown of the material. The conditions of dielectric breakdown and its applications in ophthalmic surgery have been described in the book "YAG Laser Ophthalmic Microsurgery" by Trokel. Bille et al. further discloses that the preferred method of removing tissue is to move the focused point of the surgical beam across the tissue. Near the threshold of the dielectric breakdown, the laser beam energy absorption characteristics of the tissue changes from highly transparent to strongly absorbent. The reaction is very violent, and the effects are widely variable. The amount of tissue removed is a highly non-linear function of the incident beam power. Hence, the tissue removal rate is difficult to control.
Additionally, while the technique might be useful for creating incisions within the stroma without disruption of the epithelium, Bowman's layer, or endothelium, such a procedure requires extremely precise control of the point at which the laser beam interacts with the tissue. Without such precise control, use of the procedure risks accidental exposure of the endothelium to the laser beam, and thus permanent damage to the eye. Bille fails to disclose a means by which the laser may be accurately and precisely controlled to affect only the desired tissue in the precise manner necessary for the success of such a procedure.
Currently, two methods are known by which a laser beam can be directed to specific points within the eye. One method involves manually directing a hand-held contact probe such as described in a paper entitled "Optically coupled technique for photorefractive surgery of the cornea" by J. Taboada and R. H. Poirier (Optics Letters, Vol. 15, No. 9, May 1990). Laser radiation capable of tissue photodisruption is delivered to ablate tissue within the stroma. The laser beam is delivered by means of a microscopic objective handpiece at the end of a low-inertia air-bearing-supported delivery arm. The handpiece comprises a pencil-like contact probe having objective lenses. Both lenses are plano-convex sapphire lens, with the plano side of the distal lens in contact with the corneal epithelium. The lenses function together to focus the laser beam. The physician directs the handpiece to a specified point and activates the laser. The contact probe is then repositioned and once again the laser is activated. This point-by-point process is repeated until the desired affect is achieved (e.g., an incision is created within the stroma). While this procedure provides a means to accurately affect the tissue within the stroma, it is limited in that the laser beam cannot be accurately controlled in the Z-axis (i.e., the depth of the interaction point is fixed). Moreover, control in the X and Y plane is limited by the inability of the surgeon to make straight cuts accurately by connecting contact points. Furthermore, the process is laborious and requires extraordinary care and control by the surgeon in positioning the contact probe.
The second method by which the laser beam may be directed-to specific points within the eye involves automated control of the point at which the laser is focussed. In U.S. Pat. No. 4,718,418, L'Esperance, Jr. (the '418 patent), discloses the use of a scanning laser to achieve controlled ablative photodecomposition of one or more selected regions of a cornea. According to the disclosure, a laser beam is reduced in its cross-sectional area, through a combination of optical elements, to a 0.5 mm by 0.5 mm rounded-square beam spot that is scanned over a target by deflectable mirrors. To ablate a corneal tissue surface with such an arrangement, each laser pulse etches out a square patch of tissue. The patient's head is stabilized with respect to the laser by a clamping means. An eye-retaining fixture comprising a hollow annulus, having a convergent axial-end wall of air-permeable material contoured to engage and retain the eye via the scleral-corneal region, is fixed to the patient's eye to stabilize the position of the tissue to be affected by the laser with respect to the beam control apparatus. Each such square patch must be placed precisely adjacent to the next patch; otherwise, any slight displacement of any of the etched squares would result in grooves or pits in the tissue at the locations where the squares overlap and cause excessive erosion, and ridges or bumps of unetched tissue at the locations in the tissue where the squares are not contiguous. The resulting minimum surface roughness therefore will be about two times the etch depth per pulse. A larger etch depth of 14 microns per pulse is taught for the illustrated embodiment. This larger etch depth would be expected to result in an increase of the surface roughness, thus adversely affecting visual acuity.
If this type of system were used to ablate tissue within the stroma to try to preserve a smooth epithelium, there is a high risk the procedure would fail and damage would result to the epithelium, Bowman's layer, or endothelium. This is because the typical depth of a cornea is 600 microns. Thus, even such motion of the cornea with respect to the laser source as is generated by the patient's pulse or respiration could cause the interaction point of the laser beam to unintentionally and harmfully disrupt tissue which must remain undisturbed, such as the endothelium. Furthermore, even though an eye-retaining fixture is fixed to the scleral-corneal region, and the retaining fixture has a flange which allows the eye retaining fixture to be secured to the laser, small amounts of motion of the eye retaining fixture relative to the laser source are possible. In addition, the fact that the patient's head and eye must be held absolutely motionless with respect to the laser source for the entire duration of the operation places a strain on the patient.
In response to this need to more accurately determine and control the location of the point of interaction between the laser beam and the tissue to be affected, eye tracking systems have been contemplated which track the motion of the eye with respect to the source of the laser beam. Positional feedback indicative of corneal motion relative to the laser source would be used by the laser beam control system to compensate for motion of the eye with respect to the laser beam source. Clearly, such a tracking system is complex, expensive, and poses reliability concerns.
In addition to the problem of locating the interaction point of the laser beam relative to the tissue to be affected, the laser beam control system must precisely control the movement of the interaction point of the laser beam in three dimensions in order to create incisions that follow the contour of the cornea, and to create incisions of varying depth. The requirement that the interaction point be controlled in three dimensions adds a level of complexity to the procedure.
A further problem is created by the fact that the curved surface of the cornea, in combination with the difference in the index of refraction between air and the stroma, causes the laser beam to become distorted as the laser beams passes through the boundary between air and the epithelium. This distortion can further complicate the control of the laser beam by shifting the focal point, and thus the interaction point of the laser beam.
Therefore, it would be desirable to have a method and apparatus which controls the location of the focal point of a laser beam, such that extremely accurate positioning of the interaction point of the laser with respect to the eye is possible, thereby permitting the safe use of a high power laser to affect the tissue within the stroma without risk of disrupting the epithelium, endothelium, or Bowman's layer. It would also be desirable for such a laser system to accommodate motion of the eye with respect to the laser beam source. Further, it would also be desirable to be able to controllably deform the corneal surface to improve and simplify certain types of surgical procedures. Furthermore, it would be desirable to have a method and apparatus which simplifies the laser beam control requirements such that two-dimensional control of the laser beam results in an incision which follows the contour of the cornea, or which deviates therefrom in a precise and controlled manner.
The present invention provides an ophthalmic surgical system which permit safe use of a high power laser beam to affect tissue within the stroma of a human eye. The present invention also accommodates motion of the patient's eye during the surgical procedure, while maintaining accurate location of the interaction point of the surgical laser beam. In addition, the present invention allows a three-dimensional incision to be made in the cornea of a patient's eye without the need to control the laser beam in more than two dimensions.